In recent years, as the amount of information has significantly been increased in a computer system, a large-capacity, high-speed, and random-accessible optical disc drive has come into wide use as an information data recording/playback apparatus, and optical discs such as CD-R, CD-RW, DVD-R/RW, and DVD-RAM have been used as recording media.
Such optical disc drive employs a defect detection apparatus for detecting a defect that is an area where writing or reading in/from the optical disc is not normally carried out. More specifically, when a light beam is converged and applied onto the optical disc, the defect detection apparatus detects a change in envelope of a reflection signal which is obtained in accordance with the intensity of the light reflected at the optical disc, thereby to detect a defect on the optical disc, and outputs a defect detection signal indicating whether there is a defect or not. The defect detection signal is used as a signal for previous value hold by a servo circuit that controls tracking and focusing on the optical disc, or it is used for obtaining an extraction signal that is used for judging a non-writable area on the optical disc, using a CPU that is incorporated in the optical disc drive to carry out various kinds of controls (for example, Japanese Published Patent Application No. 2003-196853).
A conventional defect detection apparatus will be described with reference to FIG. 9. FIG. 9 is a block diagram illustrating the construction of the conventional defect detection apparatus. In FIG. 9, reference numeral 1000 denotes a variable gain amplifier for amplifying, with a predetermined gain, a reflection signal AS according to the intensity of light reflected at an optical disc when a light beam is converged and applied onto the optical disc. The variable gain amplifier 1000 is supplied with a write gate signal WTGT indicating whether the optical disc drive carries out recording or playback. Reference numeral 1001 denotes a high-speed envelope detection circuit for detecting an envelope of an output signal AP of the variable gain amplifier 1000 (hereinafter referred to as an amplifier output signal). Reference numeral 1002 denotes an integration circuit for integrating an output signal EM of the high-speed envelope detection circuit 1001. Reference numeral 1003 denotes a slice level setting circuit for setting a slice level SD as a reference signal to be used in a comparator 1004 described later, on the basis of the output signal of the integration circuit 1002. Reference numeral 1004 denotes a comparator for comparing the output signal EM of the high-speed envelope detection circuit 1001 with the slice level SD, thereby to output a defect detection signal DD.
Next, the operation of the conventional defect detection apparatus constructed as described above will be described with reference to FIGS. 9 and 10. FIG. 10 shows the waveforms of the respective signals to be output in the defect detection apparatus shown in FIG. 9.
Initially, the reflection signal AS is input to the variable gain amplifier 1000. Since level of the reflection signal AS varies according to whether the optical disc is during recording or during playback, the variable gain amplifier 1000 amplifies the reflection signal AS to a predetermined amplitude, with a predetermined gain according to the operation of the optical disc drive, on the basis of the write gate signal WTGT, so as to prevent the level difference from being detected as a change in the envelope, and supplies an amplifier output signal AP to the high-speed envelope detection circuit 1001.
Next, the high-speed envelope detection circuit 1001 detects the envelope of the inputted amplifier output signal AP, and outputs an envelope signal EM to the integration circuit 1002 and the comparator 1004. The integration circuit 1002 integrates the output signal EM of the high-speed envelope detection circuit 1001, and outputs a signal IS to the slice level setting circuit 1003.
Next, the slice level setting circuit 1003 converts the level of the output signal IS of the integration circuit 1002 into a slice level SD, and outputs the slice level SD to the comparator 1004.
When there is a defect on the optical disc, usually, the levels of the reflection signal AS and the amplifier output signal AP steeply drop as shown in FIG. 10. Since the waveform of the envelope signal EM outputted from the high-speed envelope detection circuit 1001 follows the waveform of the amplifier output signal AP, the level of the envelope signal EM becomes approximately equal to the level of the amplifier output signal AP. On the other hand, since the time constant of the integration circuit 1002 is smaller than that of the high-speed envelope detection circuit 1001, the waveform of the output signal of the integration circuit 1002 does not follow the steep level drop of the envelope signal EM, but changes gently as shown in FIG. 10.
Next, the comparator 1004 compares the envelope signal EM with the slice level SD, and outputs, as a defect detection signal DD, a pulse TS1 indicating that a defect is detected, when the envelope signal EM becomes lower than the slice level SD.
However, since the gain set value of the variable gain amplifier 1000 has some degree of variation or the like, it is difficult for the variable gain amplifier 1000 to completely remove the level difference between the reflected light during recording of the reflection signal AS and the reflected light during playback thereof. Therefore, when the operation to the optical disc is changed from recording to playback or from playback to recording, there is a possibility that a level difference might occur as in the output signal AP of the variable gain amplifier 1000 shown in FIG. 10. In the conventional defect detection apparatus, such level difference causes problems as follows.
When the operation to the optical disc is changed from recording to playback, the envelope signal EM outputted from the high-speed envelope detection circuit 1001 follows the level drop of the reflection signal AS. However, the output signal IS of the integration circuit 1002, which is obtained by integrating the envelope signal EM, changes gently as shown in FIG. 10, and it takes time to follow the level drop of the reflection signal AS. Therefore, the output signal SD of the slice level setting circuit 1003 exceeds the output signal EM of the high-speed envelope detection circuit 1001, and the comparator 1004 outputs an incorrect defect detection signal FS (hereinafter referred to as “false defect signal”) immediately after the operation of the optical disc drive has changed from recording to playback although there is no defect on the optical disc.
Further, when the operation to the optical disc has changed from playback to recording, no false defect signal FS is output, but it is difficult to carry out accurate defect detection during a period tF in the defect detection signal DD shown in FIG. 10.